Coenzymic function of 1- OR N6-substituted analogs of adenosylcobalamin in the diol dehydratase reaction

Abstract

Five analogs of adenosylcobalamin modified in the adenine moiety of the Coβ ligand were synthesized, and their coenzymic properties were studied with diol dehydratase of Klebsiella pneumoniae ATCC 8724. N 6-Methyladenosylcobalamin showed a coenzymic activity of 17% that of adenosylcobalamin and a K m value of 4.2 μM. Like the natural holoenzyme, the complex between apoenzyme and this analog acted on glycerol and underwent suicide inactivation, although the rate of inactivation was much slower than that with the natural holoenzyme. Almost no spectral change was observed in the visible region during the course of the inactivation by glycerol, suggesting the formation of some new alkylcobalamin-like species as observed in the inactivation of the natural holoenzyme by glycerol. 1-Methyladenosylcobalamin and N 6-ethyladenosylcobalamin did not show detectable coenzymic activity in either 1,2-propanediol- or glycerol-dehydration reaction, although their affinity for the enzyme ( K i = 5.3 and 5.4 μM, respectively) was not so much different from that for N 6-methyladenosyleobalamin. 1, N 6-Ethenoadenosylcobalamin was also inactive and showed very low affinity for the enzyme ( K i ≈ 35 μM), indicating that simultaneous blockage of both 1- and N 6-positions inhibits the interaction with the enzyme. 3, N 4-Ethenocytidylcobalamin was inactive, although it binds to the enzyme as tightly ( K i = 1.4 μM) as does adenosylcobalamin.